1. Field of the Invention
The present invention relates to a control device for a four-wheel drive vehicle to appropriately distribute driving force generated by a driving source such as an engine.
2. Description of Related Art
Various control devices for four-wheel drive vehicles have been suggested and put into practice. For example, Japanese Unexamined Patent Application Publication (JP-A) No. 2005-289160 discloses a driving force control device for a four-wheel drive vehicle having a torque distribution mechanism capable of changing a driving force distribution ratio of front and rear wheels or a right and left driving force distribution ratio of front wheels or rear wheels. The driving force control device controls, in accordance with increase of the absolute value of a lateral acceleration signal, the driving force distribution ratio of the front and rear wheels so that the rear wheel distribution ratio becomes larger. The driving force control device also controls the right and left driving force distribution ratio of the front wheel or rear wheel so that the driving force for the turning outer wheel is larger. For, another example, according to driving force control device for a four-wheel drive vehicle disclosed in JP-A No. 2001-277882, either ones of the front and rear wheels are adopted as main driving wheels, and the others are adopted as sub-driving wheels, and the driving force control device controls driving force distributed to the sub-driving wheels by changing the engaging force of a clutch. Specifically, when the wheel speed of one of the main driving wheels is equal to or more than a first predetermined speed and the wheel speed of the other of the main driving wheels is less than the first predetermined speed, or when the average wheel speed of the main driving wheels is equal to or more than a second predetermined speed and the average wheel speed of the sub-driving wheels is less than the second predetermined speed, the engaging force of the clutch is controlled so as to distribute the driving force to the sub-driving wheels until the average wheel speed of the sub-driving wheels becomes equal to or more than a third predetermined speed which is a value equal to or more than the second predetermined speed.
However, in the control device for a four-wheel drive vehicle disclosed in JP-A No. 2005-289160 described above, a clutch element for changing the driving force distribution ratio of the right and left wheels is provided each for the right and left wheels, and therefore, vehicle behavior may be different between left and right wheels because of the mechanical difference between the clutch elements (variation of characteristics). In addition, the control device disclosed in JP-A No. 2001-277882 for performing the driving force distribution control based on the vehicle operation state such as wheel speeds is useful for improving the traction performance and the turning performance by moving torque from the main drive shaft to the sub-drive shaft. However, when the yaw behavior of a vehicle is controlled by giving yaw moment to the vehicle, the driving torque transmitted to the sub-drive shaft needs to be further redistributed by clutches which are separately provided for the right and left wheels. Accordingly, highly-accurate driving force distribution control cannot be done because of the mechanical difference between the clutch elements described in JP-A No. 2005-289160, and it may be impossible to perform intended vehicle yaw behavior control.